In recent years, filter elements for high frequency communication that have a characteristic of transmitting only electrical signals in a specified frequency band have been developed by combining a plurality of surface acoustic wave (hereinafter, called SAW) resonators that use a piezoelectric material and resonators that use piezoelectric thin film bulk waves (hereinafter, called an FBAR (Film Bulk Acoustic Resonator)). Due to being smaller in outward form size than other dielectric filters and ceramic filters and having a sharp roll-off characteristic, filter parts that employ SAW and FBAR are suited for mobile communication parts such as mobile phones for which a small size and narrow fractional bandwidth are demanded. In recent years there has been a trend of including a plurality of frequency band systems in a single mobile phone for the purpose of expanding the coverage area etc., and therefore such mobile phones include filters and duplexers appropriate for a plurality of frequencies.
A duplexer is an example of an applied part that includes a ladder-type filter in which a plurality of SAW resonators and FBARs are connected. A duplexer has a transmission/reception function. The duplexer is used in a wireless device in which the transmission signal frequency and reception signal frequency are different. A ladder-type filter in which resonators are connected serial-parallel in a ladder configuration is an example of a filter that includes SAW resonators and FBARs. Ladder-type filters can be changing the number of levels and/or changing the capacitance ratio of the resonators arranged serial-parallel. Ladder-type filters are easy to design since the insertion loss, out-of-band suppression. Another example of a filter including SAW resonators and FBARs is a lattice-type filter in which resonators are connected serial-parallel in a lattice configuration. There has been a proposal to connect a capacitor to a ladder-type filter and improve the skirt characteristic of the filter (see Japanese Patent Laid-Open No. H08-65089). The “skirt characteristic” is the sharpness of a characteristic on the high frequency side and low frequency side of the passband, and the sharper the characteristic, the more favorable the characteristic is.
FIG. 17 is a circuit diagram of a typical ladder-type filter. FIG. 18 and FIG. 19 are circuit diagrams of filters disclosed in Patent Document 1. The filters illustrated as FIG. 18 and FIG. 19 are ladder-type filters in which capacitors have been connected.
Technology for improving the skirt characteristic of a filter is important technology to particularly filters and duplexers whose transmission/reception frequency spacing is adjacent, and this technology greatly influences the performance of a communication terminal.
FIG. 20A is a circuit diagram of a ladder-type filter in which a capacitor has been connected. FIG. 20B is a frequency characteristic of the ladder-type filter illustrated as FIG. 20A. The ladder-type filter illustrated as FIG. 20A includes an FBAR filter that uses AlN. The constants of the resonators included in the ladder-type filter are as follows: capacitance ratio y=16, resonance Q=800, resonance frequency of resonators S1 to S4=1.002 GHz, resonance frequency of resonators P1 to P3=0.968 GHz, capacitance of resonators S1 and S4=6.4 pF, capacitance of resonators S2, S3, and P1 to P3=3.2 pF.
When a capacitor having a capacitance of 10 pF is connected to the ladder-type filter illustrated as FIG. 20A and the frequency characteristic is checked, a change in the skirt characteristic on the high frequency side of the passband can been seen, illustrated as FIG. 20B. Connecting a capacitor having the large capacitance of 20 pF to the ladder-type filter (illustrated as FIG. 20A) enables obtaining an even more effective result.
As the capacitance of the capacitor increases, there is a possibility that the element size of the filter will become large. An increase in the element size of the filter causes an increase in the size of a communication module including the filter and a communication apparatus including the communication module.
A process for newly forming a capacitor in the filter element becomes necessary which leads to a rise in cost.